On and above the surface of the earth, radio transmission is fairly well understood. In subsurface environments, the problem is highly complex because of the electrical properties of the subsurface media along with complex geological layering and structural features. The underground mining industry, in particular the US Bureau of Mines and its successor, the National Institute of Occupational Safety and Health (NIOSH) have conducted decades of research into underground communications. Although, improved mine operational efficiencies can results from better communications, the impetus for this research has been for post-disaster emergency communications in order for the rapid assessment of mine conditions and the location of trapped or barricaded miners. In general, this research focused on non-propagating magnetic induction communications, medium frequency communications, and UHF communications. Although the quasi-static, magnetic induction, communications works very well at short ranges (less than 1,000 feet) it is cumbersome to deploy and is unsatisfactory for longer range communication links where a highly mobile solution is sought.
The use of Low Frequency (LF) communications is common in mines with electrified railroads where FM carrier communications over the electric train trolley-wire with the rail serving as the return signal path is used. This is a highly dependable communication system and is mechanically more robust than typically wired telephone systems. The nominal carrier frequency is from 61 to 190 kHz. The disadvantage is that communications are limited to devices connected to the trolley-wire and track, which is useless for users not in proximity to a rail line.
The use of Medium Frequency (MF) communications in mines have been the subject of a large amount of research and development efforts. MF communications in mines can take advantage of existing mine infrastructure, such as rails, pipes, communication cables, etc. where the MF signals parasitically couple to these infrastructure systems. This allows extended propagation of the MF signals in a mine. Additionally, the geological nature of many coal seams have a conductive rock layer bounding the low-conductivity coal both above and below a seam. This can create a dielectric waveguide in which MF signals can readily propagate.
Very little research has been done on the use of High Frequencies (HF) for communications in mines. Common attenuation rates exceeding 70 dB/100 ft have made the use of HF frequencies unattractive for coal mine communications.
Leaky feeder mine communication systems using UHF frequencies are available commercially. A leaky feeder communication system consists of a coaxial cable, run along mine passageways, which emits and receives radio waves, functioning as an extended antenna. The cable is “leaky” in that it has gaps or slots in its outer conductor to allow the radio signal to leak into or out of the cable along its entire length. Because of this leakage of signal, line amplifiers must be inserted at regular intervals, typically every 1,000 to 5,000 feet, to amplify the signal. The signal is detected by personnel-carried portable transceivers. Transmissions from the portable transceivers are picked up by the feeder and carried to other parts of the mine, allowing two-way radio communication throughout the mine. These systems use the lower part of the UHF band (300 MHz-3 GHz) with typical operation at 350 MHz to 450 MHz. The system has a limited range and because the signal frequencies used by leaky feeder systems cannot pass through solid coal or rock; thus limiting the range of communications to line-of-sight of the leaky feeder cable.